An example eventlistener class for creating new event emitters.

eventlistener-example2.js

class EventEmitter {
  constructor() {
    /* Events object for storing event symbols */
    this._events = {};
  }
  /* Registers an event handler/listener in the Node.js style to be called once. */
  once(eventName, handler) {
    this.setupEventNameSpace(eventName);
    this.registerEventHandler(eventName, "once", handler);
    return this;
  }
  /* Registers an event handler/listener in the Node.js style to be called multiple times. */
  on(eventName, handler) {
    this.setupEventNameSpace(eventName);
    this.registerEventHandler(eventName, "on", handler);
    return this;
  }
  /* emit method for emitting events; if the symbol isn't available nothing happens. 
   * In any othercase, the functions listed under the event are executed.
   */
  emit(eventName) {
    let args = Array.prototype.slice.call(arguments);
    if(this._events[eventName] !== undefined) {
      this._events[eventName].forEach((listener, index) => {
        if(/once/.test(listener.type)) {
          console.log("Once Handler Removed");
          this._events[eventName].splice(index, 1);
        }
        this.callHandler(listener.handler, args.slice(1));
      });
    }
  }
  /* Creates an array for each events listed functions/listeners.*/
  setupEventNameSpace(eventName) {
    if(this._events[eventName] === undefined) {
      this._events[eventName] = [];
    }
  }
  /* Registers event handlers by type */
  registerEventHandler(eventName, type, handler) {
    this._events[eventName].push({ type, handler });
  }
  /* Calls an event handler, passing any extra arguments given by the emit method */
  callHandler(handler, args) {
    if(typeof handler == 'function') {
      handler.apply(this, args);
    }
  }
}